The present invention relates to an arrangement for transmission of signal units to at least one receiver.
As far as, for instance, TV-signals are concerned, these may reach viewers via terrestrial transmitters, cable networks or satellites. Each transmitted TV-channel comprises an image signal, a so-called video signal and one or more sound signals, so-called audio signals and usually a text-TV-signal. All signals are modulated onto a carrier wave. The TV-receiver, which is connected to the TV receiver antenna receives the signals. The channel selector in the TV-receiver, i.e. the unit for channel tuning in the TV-receiver, tunes into a desired TV-channel, where after the signals are detected, i.e. demodulated. The sound signal is fed to the speaker for acoustic reproduction and the image signal is fed to a decoder where after the image reproduction can take place.
For instance, cable TV-transmissions are encrypted, i.e. the respective signals characteristics is altered, with the aim of preventing unauthorised viewing, i.e. preventing image- and sound reproduction of pay-TV-channels for which no payment has been made. In order to receive such signal and reproduce it in a satisfactory manner the TV-receiver may include means, such as a separate decoder being arranged to decrypt the respective encrypted signal to its original characteristics where after image- and sound reproduction can take place in a satisfactory manner.
However, such decoder may be manipulated, whereby the image- and sound reproduction can take place in a satisfactory manner without a user having paid for the TV-channel in question.
Different transmission- and reception systems for encrypted TV-signals are known which are arranged to render unauthorised image- and sound reproduction more difficult.
EP, A2, 0 489 385 discloses one such system. The signals are encrypted in such manner that they cannot be received and decrypted by means of a regular TV-receiver, but only by means of a receiver that includes an adapted decoder, which includes a control circuit that periodically receives a secret code from a central computer via a telephone line, whereby the secret code is necessary for the decryption operation. The secret code is sent according to an encrypted format and at irregular time intervals. Furthermore, the secret code is altered with the aim at ensuring the system's security. The secret code is transmitted to the user under the condition that he/she has paid for a particular channel, for instance, during a previous payment period.
The international patent application WO, A1, 97/23996 describes a method and an apparatus for selection of programs and channels according to a specific user's preferences with respect to, for instance, avoidance of certain program types or specific programs. Alternatively, the user may choose only to have particularly offensive or objectionable portions in the programs that are transmitted to his/her receiver blocked or substituted by alternative sound- or audio sequences. The setting of the selection function is exclusively effected locally at the particular user location. The user thus has full freedom to determine what should be blocked and what should be allowed through.